AppSync Events API WebSocket fails with angle brackets in payload

WebSocket connections fail when publishing events containing angle brackets (<>)
with "BadRequestException: signature mismatch", while HTTP POST works fine.

**Steps to Reproduce:**
1. Create AppSync Events API with IAM auth
2. Publish via HTTP POST: `{"channel":"/test","events":["{\"event_1\":\"\"}"]}` ✅ Works
3. Publish via WebSocket: same payload ❌ Fails with signature error

**Expected:** Both HTTP and WebSocket should handle angle brackets consistently
**Actual:** WebSocket requires Base64 encoding as workaround

**Workaround:** Encode angle brackets as Base64 before WebSocket publish

Start by reproducing the AppSync Events API IAM-authenticated publish described in the issue, comparing the HTTP POST and WebSocket paths with angle brackets in the payload. Done means the WebSocket publish accepts the same payload as HTTP without Base64 encoding or a signature mismatch.

Written by the indexing model from the issue text.